The present invention relates to a peak-hold circuit and an infrared communication device provided with such a circuit. It further relates to the prevention of malfunction resulting from variations in the input-signal level.
FIG. 8 is a block diagram showing an electrical construction of a commonly-used infrared receiver 1. Infrared light from a transmitting device is photoelectrically transferred by a photodiode d, and inputted to amplifier a2 capable of variably gaining an ac component through pre-amplifier al and coupling capacitor c0. The output of amplifier a2 is voltage-divided by resistors r1 and r2, and then inputted to peak-hold circuit ph1. Peak-hold circuit ph1, which has a comparatively short time constant, holds the peak value of the input signal by using hold capacitor c1.
The holding value of peak-hold circuit ph1 is voltage-divided by dividing resistors r3 and r4, and inputted to the inversion input terminal of comparator cmp1. The output of amplifier a2 is applied to the non-inversion input terminal of comparator cmp1 through dividing resistors r1 and r2. The output of comparator cmp1 is applied to the base of output transistor q1. The collector of output transistor q1 is connected to a power line with xe2x80x9chigh levelxe2x80x9d Vcc through resistor r5, and is also connected to an output terminal p0, while the emitter thereof is grounded.
Moreover, the output of amplifier a2 is inputted to peak-hold circuit ph2 having a comparatively long time constant, and the holding value of hold capacitor c2 is inputted to the non-inversion input terminal of comparator cmp2. A predetermined reference voltage vref1 is applied to the inversion input terminal of the above-mentioned comparator cmp2 so that comparator cmp2 outputs an AGC signal. This increases the gain of amplifier a2 when the holding value of peak-hold circuit ph2 is lower than the reference voltage vref1 and decreases it when the holding value thereof is higher than the reference voltage vref1. Therefore, the peak level of noise of externally-applied light is captured by peak-hold circuit ph2. When the level becomes greater than the reference voltage vref1, an AGC operation for reducing the gain of amplifier a2 is carried out.
In the infrared receiver 1 having the above-mentioned construction, the photoelectrically-transferred output of the photodiode d, as shown in FIG. 9(a), is amplified by amplifiers a1 and a2 as is indicated by reference symbol xcex11 in FIG. 9(b). The holding value of peak-hold circuit ph1 is indicated by reference symbol xcex12 while the discrimination level of comparator cmp1, which is determined by divided-voltage outputs of resistors r3 and r4, is indicated by reference symbol xcex13. Therefore, comparator cmp1 level-discriminates the output of amplifier a2 by using the divided voltage values of the holding value of peak-hold circuit ph1. The results of the discrimination is inverted by output transistor q1 and resistor r5. Thus, a low-active receiving signal waveform, as shown in FIG. 9(c), is outputted to the output terminal p0.
In another situation, multiples of infrared communication devices, each of which uses the infrared receiver 1 having the above-mentioned construction, are connected in a time-sharing manner. That is, a common host device 2 and multiples of subordinate devices 3 communicate with each other, for example, as shown in FIG. 10. Supposing that the host device 2 is a receiver and one of the subordinate devices 3 is a transmitter, the light-receiving level of the host device 2 varies greatly depending on the distance and directional angle between the respective subordinate devices 3 and the host device 2.
Therefore, in the case when infrared light from a subordinate device located in a comparatively short range or on the front side of the photodiode d is switched to infrared light, from another subordinate device located in a comparatively long range or on the non-front side of the photodiode d, in response to the level variation of the receiving signal as indicated by reference symbol xcex11, the peak hold level merely follows in a manner as indicated by reference symbol xcex12, as shown in FIG. 11(a). As such, the detection level is merely allowed to follow in a manner as indicated by reference symbol xcex13. In other words, upon receiving a signal from a subordinate device whose signal level is small and which is located in a long range or on the non-front side, the detection level, which still remains great after having followed the signal level of the subordinate device located in a short range or on the front side, fails to return to a predetermined initial level L1. This results in a problem in which a discrimination error occurs in comparator cmp1, thereby causing a malfunction in the output waveform as shown in FIG. 11(b).
FIG. 12 is a block diagram which shows an electrical construction of typical prior-art peak-hold circuit ph11 which can solve the above-mentioned problem. An input signal, which has been inputted to the input terminal p1, is inputted to the non-inversion input terminal of comparator cmp11 through input resistor r11. To the inversion input terminal of this comparator cmp11 is inputted the output from the output terminal p2 of comparator cmp12 which will be described later, through feed-back resistor r12. Comparator cmp11 supplies hold capacitor c11 with a charging current through resistor r13 and diode d11 when the input signal is higher than the output signal. Discharging constant current source f11, which has a current value smaller than the charging current from comparator cmp11, is parallel-connected to hold capacitor c11. The terminal voltage of hold capacitor c11 is outputted to the output terminal p2 through the above-mentioned comparator cmp12 that functions as a buffer.
The output of the above-mentioned comparator cmp11 is also supplied to the inversion input terminal of comparator cmp13 through resistor r14, and the non-inversion input terminal of comparator cmp13 is grounded through resistor r15. Comparator cmp13 outputs xe2x80x9clow levelxe2x80x9d to capacitor c12 from its output terminal when the output of comparator cmp11 goes high. Further, the input terminal of capacitor c12 is pulled up to xe2x80x9chigh levelxe2x80x9d Vs through resistor r16. Therefore, when comparator cmp13 outputs xe2x80x9clow levelxe2x80x9d, capacitor c12 makes a discharge instantaneously, and when the output of comparator cmp13 is opened, charging is carried out in accordance with the time constant of c12xc2x7r16.
The terminal voltage of capacitor c12 is inputted to the non-inversion input terminal of comparator cmp14, and if the terminal voltage of capacitor c12 is higher than the reference voltage vref11 inputted to the inversion input terminal, comparator cmp14 outputs xe2x80x9chigh levelxe2x80x9d. If the terminal voltage is not higher, it outputs a xe2x80x9clow levelxe2x80x9d. The output from the above-mentioned comparator cmp14 is voltage-divided by resistors r17 and r18, and supplied to the base of transistor q11. The collector of transistor q11 is connected to the input terminal of the aforementioned hold capacitor c11 through resistor r19, and the emitter is grounded.
Therefore, during the period in which the output from comparator cmp14 is maintained at a xe2x80x9chigh levelxe2x80x9d, transistor q11 is parallel-connected to constant current source f11 so as to allow hold capacitor c11 to discharge, and maintained at the aforementioned initial level L1.
In the peak-hold circuit ph11 having the above-mentioned construction, in response to the input signal waveform as shown in FIG. 13(a), the output signal waveform of comparator cmp11 has a shape as indicated in FIG. 13(b), and the output signal waveform of comparator cmp13 has a shape as indicated in FIG. 13(c). Therefore, in comparator cmp14, by adjusting the time constant c12xe2x96xa1r16 as well as the reference voltage vref11, judgement timing for making a judgement that the input signal is no longer detected is delayed so that transistor q11 is allowed to conduct so as to carry out a resetting operation at time t2 at which a predetermined time period td has elapsed from time t1 when the input signal was no longer detected, as shown in FIG. 13(d). Thus, the holding value of hold capacitor c11, shown in FIG. 13(e), can be reset to the aforementioned initial level L1.
In peak-hold circuit ph11 having the above-mentioned construction, upon the resetting operation, since transistor q11 is allowed to conduct, making the charge of hold capacitor c11 discharged instantaneously, the holding value drops lower than the aforementioned initial level L1 as indicated by reference symbol xcex12 in FIG. 14(a). Here, in FIG. 14(a), the input signal is indicated by reference symbol xcex11 and the detection level is indicated by reference symbol xcex13 in the same manner as FIG. 9(b) and FIG. 11(a). Consequently, the output whose waveform is shaped by an output circuit consisting of comparator cmp1, transistor q1, etc. comes to have a shape as shown in FIG. 14(b), resulting in a problem in which error pulses are generated as shown in FIG. 14(b).
Moreover, infrared communication elements have been designed so as to be installed in portable information communication devices, and elements capable of bidirectional communication, which are integrally constituted by light receiving and emitting elements so as to allow miniaturization and cost reduction, have been developed. FIG. 15 shows a schematic construction of a bidirectional communication element 11. In this bidirectional communication element 11, a light-emitting diode that forms the transmitting end, substrate 12 on which an integrated circuit for driving the diode is installed, a diode that forms the receiving end, and a substrate 13 on which a receiving integrated circuit is installed are sealed by resin, etc. as an integral part.
In this construction, one portion of output light directed to the communication element on the other communication end, indicated by reference symbol 14, is turned around to the light-receiving element side as indicated by reference symbol 15 through the sealing resin, etc. This results in the holding value of the aforementioned peak-hold circuit ph2 rising, and the gain in amplifier a2 dropping due to the aforementioned AGC operation.
In other words, as illustrated in FIG. 16(a), even if the output of a transmitting signal is stopped and switched to the receiving operation at time t11, the holding value of peak-hold circuit ph2 still remains high as shown in FIG. 16(c), and comes to be effective in the receiving operation from time t12 at which it drops below the aforementioned reference voltage vref1. This thereby makes it possible to start a waveform-shaping operation on the received signal as shown in FIG. 16(d) in response to the transmitting signal from the other communication end as shown in FIG. 16(b). For this reason, the period between time t11 and time t12 forms dead time during which no signal is received, resulting in degradation in performance in the communication device.
Supposing that the electrostatic capacity of hold capacitor c2 is c2, a voltage rise in hold capacitor c2 due to a signal input is xcex94vc2, and the discharging current is ic, dead time toff is represented as follows:
toFF=c2xc3x97xcex94vc2/icxe2x80x83xe2x80x83(1).
The objective of the present invention is to provide a peak-hold circuit which can prevent malfunction and which has improved performances and an infrared communication device using such a peak-hold circuit.
In order to solve the above-mentioned objective, the peak-hold circuit of the present invention, which is a peak-hold circuit in which a hold means captures the peak value of an input signal and a reset means carries out a resetting operation on the holding value of the hold means when, upon switching inputs, it receives a reset signal, is designed so that the reset means, upon receipt of the reset signal, improves the response speed of the hold means by a predetermined time period.
In the above-mentioned arrangement, the charge of the hold capacitor is not discharged instantaneously by using a switching means and a resistor etc., but is discharged while improving the response speed by minimizing the time constant of the hold means.
Therefore, it becomes possible to prevent undershoot in which the holding value drops below a predetermined initial level, and consequently to prevent malfunction.
In the above-mentioned construction, the reset means is preferably designed to have constant current circuits and switching means for increasing a charging current and a discharging current of the hold means respectively. Thus, the response speed of the hold means can be improved more positively.
In order to achieve the above-mentioned objective, the infrared communication device of the present invention, which is an infrared communication device capable of time-sharing multi-channel communication that has a photoelectric transfer element for photoelectrically transferring a received infrared signal, a peak-hold circuit for capturing a peak value of an output from the photoelectric transfer element and for setting a detection level based upon the peak value and an output circuit for waveform-shaping an output from the photoelectric transfer element by level-discriminating the output based on the detection level, is provided with the peak-hold circuit of the present invention.
In the above-mentioned construction, the resetting operation of the holding value of the peak hold circuit can be carried out without undershoot. Therefore, it is possible to prevent error pulses from appearing in the output waveform detected based upon the holding value, and also to preferably receive infrared signals from a plurality of communication devices having different ranges and angles of beam spread in a time-sharing manner, upon carrying out multi-channel communication.
In order to solve the aforementioned objective, another infrared communication device of the present invention, which is a bidirectional infrared communication device having light receiving and emitting elements that are integral with each other, is provided with a time counter. The time counter, based on the fact that no level variation takes place in a transmitted signal during a predetermined period, detects completion of the transmitted signal, and allows the receiving device to restore its sensitivity.
In the above-mentioned construction, by setting the predetermined period at the maximum dead time that is determined by the communication regulation, completion of transmission can be detected, and in response to this, the holding value of the peak-hold circuit and the sensitivity of the receiving device, which have varied from the predetermined initial level due to infrared light upon transmission, can be reset, and after the lapse of the predetermined period, the receiving operation can be readily started, thereby making it possible to improve the performance.
In the above-mentioned construction, the receiving device is preferably provided with: a gain-variable amplifier for amplifying the photoelectrically transferred output from the light-receiving element; a first peak-hold circuit for carrying out a peak detection using a comparatively short time constant so as to set the detection level based upon the output of the amplifier; a second peak-hold circuit for carrying out a peak detection using a comparatively long time constant so as to achieve the AGC operation by detecting the noise level of the output of the amplifier and controlling the gain of the amplifier in response to the result of the detection; and an output circuit for waveform-shaping by level-discriminating the output of the amplifier by the detection level that has been set by the first peak-hold circuit, in which the time counter carries out a resetting operation on the gain of the amplifier by resetting at least the holding value of the second peak-hold circuit, thereby allowing the sensitivity restoration.
In the above-mentioned construction, by resetting the holding value of the second peak-hold circuit for setting the AGC level, the gain of the amplifier, which amplifies the photoelectrically transferred output from the light-receiving element, is reset so that the sensitivity of the receiving device is restored to the predetermined initial level. In addition, the holding value of the first peak-hold circuit for setting the detection level may also be reset.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.